This invention concerns the construction of threads for screw fasteners, particularly those for screwing into timber, and has particular application to fasteners for attaching railway track to timber sleepers.
Many forms of screws have been proposed and used for holding railway track to timber sleepers. They have been used with new sleepers and for insertion into holes previously created by spike fasteners, where the holes had enlarged to the point where the spikes were loose, or for so-called spike killed sleepers where no-further sound timber was available on a sleeper to drive in a further spike.
It has now been found that a screwed fastening with the novel thread characteristics described hereunder has significant advantages during installation and in track operation performance compared with existing alternatives. Although developed particularly for rail track application, the thread has much wider applications.
In one aspect the present invention provides a screw thread having a threadform comprising a ridge rising from root to crest with the crest having two peaks separated by a trough the depth of which is less than the height of the ridge from root to crest. Preferably the trough depth is between 10% and 40% of the ridge height, more preferably between 15% and 35%.
In another aspect the invention provides a screw thread having a twin start helical thread configuration consisting of two ridges helically winding around a shank, each of the ridges having flanks rising from a root to a crest, and at least one of the crests having a pair of peaks. The crests of both of the helically wound ridges may have a pair of peaks. One of the ridges may be higher than the other ridge. Preferably the lower ridge is between 30% and 70% of the height of the higher ridge.
In another aspect the invention provides a twin start screw thread for a fastener, the thread having a repeated treadform profile comprising:
a first crest, having a first pair of peaks, rising from a first root and falling to a second root, and
a second crest, having a second pair of peaks, rising from the second root and falling to the first root.
The first crest may be higher than the second crest. Preferably the second crest is between 30% and 70% of the height of the first crest.
In another aspect the invention provides a screw fastener for securing a railway track rail to a timber sleeper, the fastener having a thread as described above.
In a further aspect the invention provides a method of rolling a helical screw thread onto a cylindrical shank of a metal workpiece comprising:
rolling into the shank initial helical grooves by plastically deforming the metal into a hump immediately to either side of the grooves,
subsequently deepening and widening the initial grooves by fixer rolling which increases the size of the humps and displaces the humps increasingly further from the initial grooves,
further deepening and/or widening the grooves to press the two humps between adjacent grooves into each other until they produce a single ridge with a crest carrying a pair of peaks, and
finish rolling the crest to more uniformly define the two peaks along the crest of the ridge.
Preferably the height of each pair of peaks, when measured from a trough between aid pair, is between 10% and 40% of the height of their respective ridge. More preferably it is between 15% and 30% of the height of the ridge.
Preferably:
the screw thread has a twin start with said two initial helical grooves axially offset from each other by less than 45% (160xc2x0) of their lead,
the finished thread has the helical crests of the ridges offset by substantially 50% (180xc2x0) of their leads, and
as one helical groove is displaced axially relative to the other helical groove during the rolling process, one helical ridge is produced which is taller than the other.
More preferably the axial offset is between 20% (70xc2x0) and 35% (125xc2x0) of the thread lead.
In a further aspect the invention provides a screw fastener for securing a railway track rail to a timber sleeper, the fastener having a thread as described above. Preferably the fastener has a head for applying torque to the fastener and a collar integrally formed with the head, the collar having a sloping face facing the thread, said slope matching that of the upper surface of the foot of the rail.
The invention will now be described with reference to the attached drawings which illustrate particular embodiments of the invention.